It has been a common practice for many centuries to burn wood to heat a home or other building structure. Further, for at least more than two centuries, there have been various efforts to burn the wood more effectively. For example, in 1740, Benjamin Franklin designed his "Pennsylvania Fire-place" to extract more heat from the burning of the wood, with the hope of alleviating the problem of the then existing fuel wood shortage around Philadelphia. In Benjamin Franklin's design, the gaseous combustion products from the wood burning fire were directed first upwardly, and then downwardly around a heat exchange apparatus, after which the gaseous exhaust was discharged upwardly through a chimney. At the same time, air was circulated from the cellar upwardly through a heat exchange apparatus to heat the room. Even at that time, Benjamin Franklin recognized that permitting too much ambient air to go up the chimney without contributing to the combustion was wasteful of the heat energy. He thus provided a shutter that slid in grooves to limit the draft of air into the wood burning chamber. He also recognized that a "dirty" fire was less efficient in producing heat.
In 1836, Isaac Orr patented his "airtight" stove. By controlling the amount of air which is drawn into the wood burning chamber, the unnecessary loss of hot air flowing up the chimney could be alleviated to some extent. Since that time, there have been hundreds, if not thousands, of proposals and designs to promote more complete combustion of the wood products, as well as to promote more effective heat exchange of the combustion products (i.e. preventing the heat from simply going up the chimney).
There are other problems which might be considered as inherent in the burning of wood. For instance, wood that is considered quite dry (e.g. split logs that have been air dried for a number of months in warm summer weather) can still contain as much as 20% water by weight. In the burning process, this moisture evaporates, and the passage of this evaporated moisture up the chimney represents lost heat.
Another complicating factor is that about half of the potential heat value in the wood is in the form of "volatiles", which are combustible gasses which are given off when the wood gets hot. When a piece of wood is placed in the wood burning chamber of a fireplace, as the temperature of the wood rises, first the water in the wood is evaporated in the form of steam, and some volatiles may be evaporated. When the wood reaches the temperature of about 300.degree.-400.degree. F., the wood begins to break down chemically, and this generates yet more volatiles in the form of gaseous organic molecules. Unfortunately, complete combustion of these volatiles can be accomplished in normal circumstances only if the temperature could rise to as high as 1000.degree. F. However, this is not always practical for a number of reasons.
There have been various attempts to improve the combustion of volatiles, and many of these have focused on what can be termed "secondary combustion". For example, the gaseous combustion products emanated from the location of the burning wood are redirected into a secondary zone where additional ambient air is introduced to enhance combustion of the volatiles.
Further, in the common wood burning stove that is used in normal conditions in a person's home, there are certain practical difficulties in attempting to "fine tune" the operation of the stove to extract the heat effectively. For example, the person burning wood as a source of heat does not want to be constantly attending the wood stove by feeding in only the amount of wood which is needed to produce the desired amount of heat for a short period of time. Rather, the person will generally stack up the wood in the fireplace and then leave the stove to burn for possibly several hours. However, if the stove is permitted to burn "wide open", heat may be generated too rapidly, thus raising the room temperature to an undesirably high level. This problem is commonly solved by regulating the draft of the stove to limit the amount of air which enters the stove. In some stoves, the flow of air into the stove can be regulated automatically, such as by use of a temperature sensitive control device. One such device is the bimetallic strip used in an automatic draft control device, this being developed by Elisha Foote of Geneva, N.Y. in approximately 1872.
However, when the draft is limited, this necessarily reduces the amount of oxygen available for complete combustion of the volatiles, and these volatiles pass unburned up the chimney. As the volatiles cool as they pass up the flue or chimney, some of these become deposited on the inside surfaces of the flue or chimney in the form of creosote. It sometimes happens that when the fire is later burning at a very high temperature, high temperature combustion products pass up the flue to ignite the accumulated creosote, thus resulting in what is commonly called a "chimney fire".
Another problem which is related to the problems noted above, and which has become more serious in recent years is the pollution which can be generated by wood burning stoves. The partially burned hydrocarbons not only introduce toxic waste into the air, but also emit highly visible smoke which is aesthetically unpleasing as it is environmentally hazardous. One approach to this problem is to burn the wood, including the volatiles, more completely. However, as indicated above, this has certain practical problems. Another approach to alleviate this problem has been to utilize catalytic converters to limit the quantity of these emissions. However, in order to avoid catalytic fouling, these catalytic converters are commonly bypassed during the initial startup of the stove until the firebox temperature reaches approximately 500.degree. F. Unfortunately, it is during this startup period that the greatest quantity of contaminants are discharged on a per hour basis. In addition, catalytic converters have a limited useful life span and must therefore be replaced periodically.
Another consideration is that it is not practical to direct the emissions from a wood burning stove through various conduits and treatment sections to process the gaseous exhaust. For a wood burning stove to operate effectively, it must "draw" adequately (i.e. there must be a sufficiently strong flow of gaseous material up the flue or chimney). If this is not accomplished, when the person opens the stove to insert more wood, there is sometimes a puff or strong flow of a relatively large volume of smoke out of the door of the firebox and into the room.
With regard to the broad subject of treating gaseous effluent of various sorts to remove contaminants, recover certain ingredients, etc., there have been various avenues that have been explored over the decades. One of these is the use of electrostatic precipitators which have been commonly used to remove particulate matter from gaseous emissions of industrial plants and other installations. Commonly, the emissions are passed between a pair of electrodes, one of which is usually grounded. The other electrode is charged to a higher negative voltage relative to the grounded electrode so that an electric field is established between the two electrodes; the electric field being sufficiently strong to ionize the emissions between the electrodes, a phenomenon known as "corona discharge". The corona discharge from the negatively charged electrode imparts a negative charge to many of the particulate emissions passing between the electrodes, which are then caused to migrate under the force of the electrostatic field toward the grounded electrode. The particulate emissions collected on the grounded electrode are then removed therefrom in some suitable manner.
There are various types of conventional electrostatic precipitators. One relatively simple precipitator includes a grounded electrode in the form of two parallel spaced plates with negatively charged wires spaced therebetween, and wherein gaseous effluents are passed between the area defined by the two plates. Another type of electrostatic precipitator comprises a grounded electrode configured as a cylinder with a negatively charged electrode comprising a wire which extends along the axial centerline of the tubular cylindrical electrode, and wherein charged particulate emissions are conducted onto the inner surface of the outer tubular electrode by an electrostatic field induced between the wire cathode and the tubular anode.
Another prior art electrostatic precipitator is disclosed in U.S. Pat. No. 4,194,888--Schwab et al, wherein a grounded electrode is configured as a cylinder with the negatively charged electrode comprising an inner elongated support electrode connected to one or more disc-shaped discharge electrodes having a transverse dimension larger than the transverse dimension of the support electrode. The disc electrodes are spaced apart along a support electrode to provide multiple charging and collection zones through which the emissions sequentially pass.
In U.S. Pat. No. 4,110,086--Schwab et al, there is disclosed an electrostatic precipitator for removing contaminants from gaseous streams wherein the electrostatic precipitator includes a central disc electrode located concentrically within a venturi throat, the venturi acting as an outer electrode.
In U.S. Pat. No. 3,656,441--Grey et al, there is disclosed an incinerator and two separating chambers to remove the contaminants from the incinerator emission gasses, and a liquid-wash medium which washes the inner walls of the first separating chamber to remove contaminants precipitated thereon. The liquid-wash medium also cools the emission products to a temperature of between 400.degree.-600.degree. F. to reduce the concentration of gaseous contaminants prior to passing the emission products through a second separating chamber. Electrostatic precipitation takes place within the second separating chamber, the second separating chamber subject to water wash down on the inner walls thereof to remove collected contaminants.
In U.S. Pat. No. 1,884,085--Miller, there is disclosed an electrostatic precipitator for removing certain constituents of emission products from a coke oven wherein a precipitation zone is heated to an elevated temperature by circulating a heating medium about an electrode tube. The emission products are maintained therein at a temperature which permits certain components to remain in the gaseous phase while causing other components to condense.
In U.S. Pat. Nos. 1,895,676--Miller, and 1,826,428, there is disclosed apparatus similar to that disclosed in the above described Miller Patent, U.S. Pat. No. 1,884,085.
In U.S. Pat. No. 4,289,504--Steere et al, there is disclosed apparatus for removing wax-like particulate matter from emissions wherein electrical heating means are provided in the outer surface of a collecting box to maintain the fluidity of normally non-fluid material removed from the emissions. Other U.S. patents disclosing electrostatic precipitators used to separate certain constituents from emissions including U.S. Pat. Nos. 3,656,440--Grey et al; 2,722,283--Klemperer et al; 2,711,225--Armstrong et al; 1,473,806--Bradley; and 1,393,712--Steere et al.